This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 1021010 filed in The Netherlands on July Jul. 5, 2002, which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to a method of printing a receiving material with hot melt ink which includes the steps of heating the ink to above a temperature at which it is liquid, imagewise transferring the liquid ink to an intermediate element using an inkjet printhead, the intermediate element having a surface containing an elastomer with a surface tension having a polar part equal to or less than 20 mN/m, and bringing the receiving material into contact with the intermediate element in such a manner that the ink transfers from the intermediate element to the receiving material. The present invention also relates to an inkjet printer suitable for applying the present method, the combination of such a printer with an ink pre-eminently suitable for such a purpose, and a method of selecting an elastomer suitable for use in such a method.
2. Related Art
General methods and printers of this type are known from U.S. Pat. No. 5,372,852. In this method, hot melt ink, i.e. an ink which is solid at room temperature but liquid at elevated temperature, is applied to the receiving material via an indirect process. For this purpose, the ink is first heated in the inkjet printhead to a temperature at which it is liquid, i.e. has a consistency such that it can be ejected in the form of small drops by means of an inkjet printhead. Printheads of this kind are well known, for example from EP 0 443 628 or EP 1 022 140. The ink drops are ejected image-wise onto a liquid intermediate surface, particularly a surface of silicone oil, present in the form of a thin layer on the surface of the intermediate element. Since the temperature of the intermediate element is much lower than that at which the ink is liquid, the ink solidifies on the intermediate element and passes into a solid but malleable state which makes the ink pressure-transferable. The ink is then brought into contact with the receiving material in a transfer nip, which is formed at the interface of the intermediate element and a pressure roller in contact therewith. As a result of the high pressure at the transfer nip, typically 750-850 p.s.i. (52-59 bar) the solidified ink is transferred from the intermediate element to the receiving material and forms a connection with that material. After further cooling to room temperature, the ink finally sits solidly on the receiving material and is reasonably resistant to mechanical action such as folding and scratching. In this known method, which is incidentally also described in U.S. Pat. No. 5,389,958, U.S. Pat. No. 5,614,933 and U.S. Pat. No. 5,777,650, it has been found very important that the surface of the intermediate element should be sufficiently rigid and hard to enable the ink present on its surface to be able to deform when the receiving material passes through the transfer nip. If the pressure is too low, the transfer yield is insufficient, resulting in poor image quality and soiling of the intermediate element with non-transferred ink. Accordingly, in the known method, a surface of anodised aluminium, which is a rigid and hard material, is preferably used, so that a high nip pressure can be achieved. It is described that elastomers such as silicone rubber, fluorosilicone rubber and Teflon can also be used. It is well known that materials of this kind generally have a low surface tension, typically lower than 50 mN/m and are mainly built up from a-polar interactions, so that these materials have relatively good release properties. It is also described that these elastomers, if they are to function in the indirect inkjet process, must satisfy the same mechanical requirements, i.e. have a rigidity and hardness comparable to that of anodised aluminium.
The known method has a number of disadvantages. Since it is necessary to obtain a high nip pressure, the intermediate element is constructed in the form of a rigid drum with a hard surface. A drum of this kind is not only expensive to produce, but also occupies relatively considerable space (as a result of the relatively large fixed diameter), particularly when it is required to use larger receiving material formats. It is very disadvantageous that this drum must be mechanically very rigidly suspended so that the really high pressures can be achieved. A suspension of this kind is expensive. Moreover, no matter how rigid the drum is, it will always sag in the middle to some extent, resulting in poor transfer. This can be obviated by making even more rigid drums or drums, for example, of different form, for example with curved surfaces, but this results in even higher production costs. As a result, the length of the drum is limited to about 13″(about 33 cm) for economically viable applications. As a result, receiving materials wider than about 12½″ cannot be printed. Another disadvantage of the use of a rigid intermediate element is noticeable when the receiving material enters the transfer nip. Despite the fact that the receiving material always has a low weight, the momentum of this material is relatively considerable due to its high speed. As a result, an entry of this kind has a noticeable impact on the drum and the pressure roller together forming the nip. This impact results in a short disturbance in the rotation of the drum and this is also perceptible at the inkjet printhead because of the high rigidity of this drum. As a result, registration errors may arise and, accordingly, the printed image may be distorted.
Another important disadvantage of the known method is that a thin layer of oil has to be applied to the intermediate element. This oil is required to enable the ink to be transferred. Without oil, the ink will hardly be transferred, if at all, to the receiving material but will adhere firmly to the intermediate element. A metering station is necessary to meter a thin layer of oil. This again increases the production costs of the printer. Furthermore, a layer of oil of this kind results in soiling of the receiving material and the printer interior. The receiving materials may as a result have a spotted impression and the printer may ultimately soil, creating a negative effect on its operation. Such soiling leads to extra maintenance costs. Another disadvantage of the use of oil is that it has to be repeatedly replaced, thus adversely affecting printer productivity.